The main objective of this study is the investigation of the possible uses of glycoproteins and carbohydrates as recognizable carriers for the selective uptake of chemotherapeutic agents by tumor cells. It is known that the liver can rapidly clear desialated glycoproteins from the circulation and that cell surface receptors showing specificity for terminal galactosyl residues are responsible for initiating this phenomenon. The receptors may be enzymatic in nature and the activities of certain glycosyltransferase and glycosidase enzymes, which are capable of interacting with terminal glycosyl residues of glycoproteins, have been assayed in both normal rat liver, hepatoma and leukemic cell fractions. Homogenates of Reuber H-35 and Novikoff hepatoma grown i.m. in rats contained less sialyltransferase activity than normal liver even though serum sialyltransferase levels were slightly higher in rats bearing Reuber hepatoma and much higher (6-fold) in rats bearing Novikoff hepatoma. An 8-fold increase in serum sialyltransferase activity was also measured in mice bearing L1210 leukemia. A parallel increase in serum glutamic-pyruvic transaminase, glutamic-oxaloacetic transminase and lactate dehydrogenase was found in mice serum on days four, five and six after the i.p. inoculation of 10 to the 5th power L1210 cells. Infiltration of the liver by leukemic cells was also evident by histologic examination on days five and six. Therefore increases in serum sialyltransferase activity in tumor bearing animals are probably the result of host liver damage. Characterization of sialyltransferase activity from liver and hepatoma tissue showed that uridine nucleotides (1.0 - 1.5 mM) enhance enzyme activity allosterically and cytidine nucleotides (apparent Ki equals 0.62 mM) competitively inhibit sialyltransferase activity. Therefore sialyltransferase activities in vivo may be regulated by pyrimidine nucleotide levels.